A problem with prior art pumps used in HPLC applications has been that there may occur flow fluctuations ("flow spikes") in the system. These "spikes" (positive or negative) are due to the delayed delivery from each pump half because of the need to build up the pressure in the cylinder to correspond to the system pressure. This pressure build-up effect comes from the finite compressibility of liquids, and inherent elasticity in the construction elements of the system. In other words, when one pump cylinder "phases out", i.e. when the piston approaches its end point during the delivery phase, and the flow is beginning to drop to zero, the other pump cylinder taking over needs some time before it can begin to deliver a flow, because of the mentioned need to generate the system pressure level in the liquid inside the cylinder. Thus, it would be desirable if the two cylinders could be synchronized in their respective "phasing in" and "phasing out", such that the flow is maintained constant during this critical phase, and suitably such that the synchronization is adaptable to different system pressure levels. Thus, there exists a need for pumps having variably controllable cam profiles that in addition adapt to the situation at hand.
EP-0 050 296 B discloses a pulsation-free volumetric pump having two plungers reciprocated by a cam so as to provide a combined discharge volume. The pump is characterized by having a DC motor having a mechanical time constant below 12 ms, and by having means for detecting pressure pulsations produced during pumping.
EP-0 334 994 A1 discloses a reciprocating type fluid delivery pump having a drive motor and plungers for driving two pump heads. The pump comprises a converting mechanism for converting rotational motion to a reciprocating motion, including a cam for each plunger. The cams are mounted on a common cam shaft rotating at constant velocity. The cams are machined to have profiles that determine the angle-plunger speed characteristics.
The driving speed is controlled by measuring system pressure and the flow in the system is thereby controllable to a certain extent.
DE-38 37 325 C2 discloses a liquid delivery plunger type pump having a main cylinder and an auxiliary cylinder, both being operated by cams mounted on a common cam shaft.
The pressure is measured and the measurements are used to provide an essentially constant flow.
DE-41 30 295 A1 discloses a pump system having separately driven plunger pumps. The rate of the individual pumps is controlled by feeding back measurement data relating to rotation speed and rotor position. System pressure is not used as a pump control parameter. The pump is said to be suitable for viscous liquids or pastes.